Method for detection of caries

ABSTRACT

A method for forming an enhanced image of tooth tissue for caries detection obtains fluorescence and reflectance image data from a tooth. Each pixel in the fluorescence image data is combined with its corresponding pixel in the reflectance image data by subtracting an offset to the reflectance image data value to generate an offset reflectance image data value, and then computing an enhanced image data value according to the difference between the fluorescence image data value and the offset reflectance image data value, whereby the enhanced image is formed from the resulting pixel array of enhanced image data values.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of U.S. patent application Ser. No. 11/468,883,filed, Aug. 31, 2006, entitled METHOD FOR DETECTION OF CARIES, by Wonget al., the disclosure of which is incorporated herein.

Reference is made to commonly-assigned copending U.S. patent applicationSer. No. 11/262,869, filed Oct. 31, 2005, entitled METHOD AND APPARATUSFOR DETECTION OF CARIES, by Wong et al., the disclosure of which isincorporated herein.

FIELD OF THE INVENTION

This invention generally relates to a method and apparatus for dentalimaging and more particularly to an improved method for early detectionof caries using fluorescence and scattering of light.

BACKGROUND OF THE INVENTION

In spite of improvements in detection, treatment, and preventiontechniques, dental caries remain a widely prevalent condition affectingpeople of all age groups. If not properly and promptly treated, cariescan lead to permanent tooth damage and even to loss of teeth.

Traditional methods for caries detection include visual examination andtactile probing with a sharp dental explorer device, often assisted byradiographic (x-ray) imaging. Detection using these methods can besomewhat subjective, varying in accuracy due to many factors, includingpractitioner expertise, location of the infected site, extent ofinfection, viewing conditions, accuracy of x-ray equipment andprocessing, and other factors. There are also hazards associated withconventional detection techniques, including the risk of damagingweakened teeth and spreading infection with tactile methods as well asexposure to x-ray radiation. By the time caries are evident under visualand tactile examination, the disease is generally in an advanced stage,requiring a filling and, if not timely treated, possibly leading totooth loss.

In response to the need for improved caries detection methods, there hasbeen considerable interest in improved imaging techniques that do notemploy x-rays. One method that has been commercialized employsfluorescence, caused when teeth are illuminated with high intensity bluelight. This technique, termed quantitative light-induced fluorescence(QLF), operates on the principle that sound, healthy tooth enamel yieldsa higher intensity of fluorescence under excitation from somewavelengths than does de-mineralized enamel that has been damaged bycaries infection. The strong correlation between mineral loss and lossof fluorescence for blue light excitation is then used to identify andassess carious areas of the tooth. A different relationship has beenfound for red light excitation, a region of the spectrum for whichbacteria and bacterial by-products in carious regions absorb andfluoresce more pronouncedly than do healthy areas.

Among proposed solutions for optical detection of caries are thefollowing:

-   U.S. Pat. No. 4,515,476 (Ingmar) discloses use of a laser for    providing excitation energy that generates fluorescence at some    other wavelength for locating carious areas.-   U.S. Pat. No. 6,231,338 (de Josselin de Jong et al.) discloses an    imaging apparatus for identifying dental caries using fluorescence    detection.-   U.S. Patent Application Publication No. 2004/0240716 (de Josselin de    Jong et al.) discloses methods for improved image analysis for    images obtained from fluorescing tissue.-   U.S. Pat. No. 4,479,499 (Alfano) describes a method for using    transillumination to detect caries based on the translucent    properties of tooth structure.

Among commercialized products for dental imaging using fluorescencebehavior is the QLF Clinical System from Inspektor Research Systems BV,Amsterdam, The Netherlands. Using a different approach, the DiagnodentLaser Caries Detection Aid from KaVo Dental Corporation, Lake Zurich,Ill., detects caries activity monitoring the intensity of fluorescenceof bacterial by-products under illumination from red light.

U.S. Patent Application Publication No. 2004/0202356 (Stookey et al.)describes mathematical processing of spectral changes in fluorescence inorder to detect caries in different stages with improved accuracy.Acknowledging the difficulty of early detection when using spectralfluorescence measurements, the '2356 Stookey et al. disclosure describesapproaches for enhancing the spectral values obtained, effecting atransformation of the spectral data that is adapted to the spectralresponse of the camera that obtains the fluorescent image.

While the disclosed methods and apparatus show promise in providingnon-invasive, non-ionizing imaging methods for caries detection, thereis still room for improvement. One recognized drawback with existingtechniques that employ fluorescence imaging relates to image contrast.The image provided by fluorescence generation techniques such as QLF canbe difficult to assess due to relatively poor contrast between healthyand infected areas. As noted in the '2356 Stookey et al. disclosure,spectral and intensity changes for incipient caries can be very slight,making it difficult to differentiate non-diseased tooth surfaceirregularities from incipient caries.

Overall, it is well-recognized that, with fluorescence techniques, theimage contrast that is obtained corresponds to the severity of thecondition. Accurate identification of caries using these techniquesoften requires that the condition be at a more advanced stage, beyondincipient or early caries, because the difference in fluorescencebetween carious and sound tooth structure is very small for caries at anearly stage. In such cases, detection accuracy using fluorescencetechniques may not show marked improvement over conventional methods.Because of this shortcoming, the use of fluorescence effects appears tohave some practical limits that prevent accurate diagnosis of incipientcaries. As a result, a caries condition may continue undetected until itis more serious, requiring a filling, for example.

Detection of caries at very early stages is of particular interest forpreventive dentistry. As noted earlier, conventional techniquesgenerally fail to detect caries at a stage at which the condition can bereversed. As a general rule of thumb, incipient caries is a lesion thathas not penetrated substantially into the tooth enamel. Where such acaries lesion is identified before it threatens the dentin portion ofthe tooth, remineralization can often be accomplished, reversing theearly damage and preventing the need for a filling. More advancedcaries, however, grows increasingly more difficult to treat, most oftenrequiring some type of filling or other type of intervention.

In order to take advantage of opportunities for non-invasive dentaltechniques to forestall caries, it is necessary that caries be detectedat the onset. In many cases, as is acknowledged in the '2356 Stookey etal. disclosure, this level of detection has been found to be difficultto achieve using existing fluorescence imaging techniques, such as QLF.As a result, early caries can continue undetected, so that by the timepositive detection is obtained, the opportunity for reversal usinglow-cost preventive measures can be lost.

Thus, it can be seen that there is a need for a non-invasive,non-ionizing imaging method for caries detection that offers improvedaccuracy for detection of caries, particularly in its earlier stages.

SUMMARY OF THE INVENTION

The present invention provides a method for forming an enhanced image ofa tooth.

It is a feature of the present invention that it utilizes bothfluorescence and reflectance image data for dental imaging.

It is an advantage of the present invention that it offers enhancementover existing fluorescence imaging techniques, useful for detection ofcaries in its incipient stages.

These and other objects, features, and advantages of the presentinvention will become apparent to those skilled in the art upon areading of the following detailed description when taken in conjunctionwith the drawings wherein there is shown and described an illustrativeembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic block diagram of an imaging apparatus for cariesdetection according to one embodiment;

FIG. 2 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment;

FIG. 3 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment;

FIG. 4A is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using polarized light;

FIG. 4B is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using a polarizingbeamsplitter to provide polarized light and to minimize specularreflection;

FIG. 5 is a view showing the process for combining dental image data togenerate a fluorescence image with reflectance enhancement according tothe present invention;

FIG. 6 is a composite view showing the contrast improvement of thepresent invention in a side-by-side comparison with conventional visualand fluorescence methods;

FIG. 7 is a block diagram showing a sequence of image processing forgenerating an enhanced threshold image according to one embodiment;

FIG. 8 is a schematic block diagram of an imaging apparatus for cariesdetection according to an alternate embodiment using multiple lightsources;

FIG. 9 is a plan view comparing results from scalar multiplication andasymmetric illuminance methods of the present invention;

FIG. 10 shows graphs for input/output pixel mapping of code valueswithout any image modification and for pixel mapping with an appliedoffset as used in one embodiment;

FIG. 11 is a plan view comparing results from scalar multiplication anddownshifting methods of the present invention; and

FIG. 12 is a plan view showing an example display with white light andenhanced images displayed for a tooth according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

As noted in the preceding background section, it is known thatfluorescence can be used to detect dental caries using either of twocharacteristic responses: First, excitation by a blue light sourcecauses healthy tooth tissue to fluoresce in the green spectrum.Secondly, excitation by a red light source can cause bacterialby-products, such as those indicating caries, to fluoresce in the redspectrum.

In order for an understanding of how light is used in the presentinvention, it is important to give more precise definition to the terms“reflectance” and “back-scattering” as they are used in biomedicalapplications in general and, more particularly, in the method andapparatus of the present invention. In broadest optical terminology,reflectance generally denotes the sum total of both specular reflectanceand scattered reflectance. (Specular reflection is that component of theexcitation light that is reflected by the tooth surface at the sameangle as the incident angle.) In many biomedical applications, however,as in the dental application of the present invention, the specularcomponent of reflectance is of no interest and is, instead, generallydetrimental to obtaining an image or measurement from a sample. Thecomponent of reflectance that is of interest for the present applicationis from back-scattered light only. Specular reflectance must be blockedor otherwise removed from the imaging path. With this distinction inmind, the term “back-scattered reflectance” is used in the presentapplication to denote the component of reflectance that is of interest.“Back-scattered reflectance” is defined as that component of theexcitation light that is elastically back-scattered over a wide range ofangles by the illuminated tooth structure. “Reflectance image” data, asthis term is used in the present invention, refers to image dataobtained from back-scattered reflectance only, since specularreflectance is blocked or kept to a minimum. In the scientificliterature, back-scattered reflectance may also be referred to asback-reflectance or simply as back-scattering. Back-scatteredreflectance is at the same wavelength as the excitation light.

It has been shown that light scattering properties differ betweenhealthy and carious dental regions. In particular, reflectance of lightfrom the illuminated area can be at measurably different levels fornormal versus carious areas. This change in reflectance, taken alone,may not be sufficiently pronounced to be of diagnostic value whenconsidered by itself, since this effect is very slight, althoughdetectable. For more advanced stages of caries, for example,back-scattered reflectance may be less effective an indicator than atearlier stages.

In conventional fluorescence measurements such as those obtained usingQLF techniques, reflectance itself is an effect that is avoided ratherthan utilized. A filter is usually employed to block off all excitationlight from reaching the detection device. For this reason, the slightbut perceptible change in back-scattered reflectance from excitationlight has received little attention for diagnosing caries.

The inventors have found, however, that this back-scattered reflectancechange can be used in conjunction with the fluorescent effects to moreclearly and more accurately pinpoint a carious location. Moreover, theinventors have observed that the change in light scattering activity,while it can generally be detected wherever a caries condition exists,is more pronounced in areas of incipient caries. This back-scatteredreflectance change is evident at early stages of caries, even whenfluorescent effects are least pronounced.

The present invention takes advantage of the observed back-scatteringbehavior for incipient caries and uses this effect, in combination withfluorescence effects described previously in the background section, toprovide an improved capability for dental imaging to detect caries. Theinventive technique, hereafter referred to as fluorescence imaging withreflectance enhancement (FIRE), not only helps to increase the contrastof images over that of earlier approaches, but also makes it possible todetect incipient caries at stages where preventive measures are likelyto effect remineralization, repairing damage done by the cariesinfection at a stage well before more complex restorative measures arenecessary. Advantageously, FIRE detection can be accurate at an earlierstage of caries infection than has been exhibited using existingfluorescence approaches that measure fluorescence alone.

Imaging Apparatus

Referring to FIG. 1, there is shown an imaging apparatus 10 for cariesdetection using the FIRE method in one embodiment. A light source 12directs an incident light, at a blue wavelength range or other suitablewavelength range, toward tooth 20 through an optional lens 14 or otherlight beam conditioning component. The tooth 20 may be illuminated at aproximal surface (as shown) or at an occlusal surface (not shown). Twocomponents of light are then detected by a monochrome camera 30 througha lens 22: a back-scattered light component having the same wavelengthas the incident light and having measurable reflectance; and afluorescent light that has been excited due to the incident light. ForFIRE imaging, specular reflection causes false positives and isundesirable. To minimize specular reflection pick up, the camera 30 ispositioned at a suitable angle with respect to the light source 12. Thisallows imaging of back-scattered light without the confounding influenceof a specularly reflected component.

In the embodiment of FIG. 1, monochrome camera 30 has color filters 26and 28. One of color filters 26 and 28 is used during reflectanceimaging, the other is used during fluorescence imaging. A processingapparatus 38 obtains and processes the reflectance and fluorescenceimage data and forms a FIRE image 60. FIRE image 60 is an enhanceddiagnostic image that can be printed or can appear on a display 40. FIREimage 60 data can also be transmitted to storage or transmitted toanother site for display.

Referring to FIG. 2, there is shown an alternate embodiment using acolor camera 32. With this arrangement, auxiliary filters would notgenerally be needed, since color camera 32 would be able to obtain thereflectance and fluorescence images from the color separations (alsocalled color planes) of the full color image of tooth 20.

Light source 12 is typically centered around a blue wavelength, such asabout 405 nm in one embodiment. In practice, light source 12 could emitlight ranging in wavelength from an upper ultraviolet range to a deeperblue, between about 300 and 500 nm. Light source 12 can be a laser orcould be fabricated using one or more light emitting diodes (LEDs).Alternately, a broadband source, such as a xenon lamp, having asupporting color filter for passing the desired wavelengths could beused. Lens 14 or other optical element may serve to condition theincident light, such as by controlling the uniformity and size of theillumination area. For example, a diffuser 13, shown as a dotted line inFIG. 2, might be used before or after lens 14 to smooth out the hotspots of an LED beam. The path of illumination light might include lightguiding or light distributing structures such as optical fibers or aliquid light guide, for example (not shown). Light level is typically afew milliwatts in intensity, but can be more or less, depending on thelight conditioning and sensing components used.

Referring to FIG. 3, the illumination arrangement could alternatelydirect light at normal incidence, turned through a beamsplitter 34.Camera 32 would then be disposed to obtain the image light that istransmitted through beamsplitter 34. Other options for illuminationinclude multiple light sources directed at the tooth with angularincidence from one or more sides. Alternately, the illumination mightuse an annular ring or an arrangement of LED sources distributed about acenter such as in a circular array to provide light uniformly frommultiple angles. Illumination could also be provided through an opticalfiber or fiber array.

The imaging optics, represented as lens 22 in FIGS. 1-3, could includeany suitable arrangement of optical components, with possibleconfigurations ranging from a single lens component to a multi-elementlens. Clear imaging of the tooth surface, which is not flat but can haveareas that are both smoothly contoured and highly ridged, requires thatimaging optics have sufficient depth of focus. Preferably, for optimalresolution, the imaging optics provide an image size that substantiallyfills the sensor element of the camera. Telecentric optics areadvantaged for lens 22, providing image-bearing light that is not highlydependent on ray angle.

Image capture can be performed by either monochrome camera 30 (FIG. 1)or color camera 32 (FIG. 2). Typically, camera 30 or 32 employs a CMOSor CCD image sensor. The monochrome version would typically employ aretractable spectral filter 26, 28 suitable for the wavelength ofinterest. For light source 12 having a blue wavelength, spectral filter26 for capturing reflectance image data would transmit predominatelyblue light. Spectral filter 28 for capturing fluorescence image datawould transmit light at a different wavelength, such as predominatelygreen light. Preferably, spectral filters 26 and 28 are automaticallyswitched into place to allow capture of both reflectance andfluorescence images in very close succession. Both images are obtainedfrom the same position to allow accurate registration of the image data.

Spectral filter 28 would be optimized with a pass-band that capturesfluorescence data over a range of suitable wavelengths. The fluorescenteffect that has been obtained from tooth 20 can have a relative broadspectral distribution in the visible range, with light emitted that isoutside the wavelength range of the light used for excitation. Thefluorescent emission is typically between about 450 nm and 650 nm, whilegenerally peaking in the green region, roughly from around 500 nm toabout 600 nm. Thus a green light filter is generally preferred forspectral filter 28 in order to obtain this fluorescence image at itshighest energy levels. However, other ranges of the visible spectrumcould also be used in other embodiments.

In a similar manner, spectral filter 26 would be optimized with apass-band that captures reflectance data over a wavelength rangecovering at least a significant portion of the spectral energy of thelight source 12 used. For reasons previously discussed, a blue lightfilter is generally used for spectral filter 26 in order to obtain thereflectance image at its highest energy level.

Camera controls are suitably adjusted for obtaining each type of image.For example, when capturing the fluorescence image, it is necessary tomake appropriate exposure adjustments for gain, shutter speed, andaperture, since this image may not be intense. When using color camera32 (FIG. 2), color filtering is performed by the color filter arrays onthe camera image sensor. The reflectance image is captured in the bluecolor plane; simultaneously, the fluorescence image is captured in thegreen color plane. That is, a single exposure captures bothback-scattered reflectance and fluorescence images.

Processing apparatus 38 is typically a computer workstation but may, inits broadest application, be any type of control logic processingcomponent or system that is capable of obtaining image data from camera30 or 32 and executing image processing algorithms upon that data togenerate the FIRE image 60 data. Processing apparatus 38 may be local ormay connect to image sensing components over a networked interface.

Referring to FIG. 5, there is shown, in schematic form, how the FIREimage 60 is formed according to the present invention. Two images oftooth 20 are obtained, a green fluorescence image 50 and a bluereflectance image 52. As noted earlier, it must be emphasized that thereflectance light used for reflectance image 52 and its data is fromback-scattered reflectance, with specular reflectance blocked or kept aslow as possible. In the example of FIG. 5, there is a carious region 58,represented in phantom outline in each of images 50, 52, and 60, thatcauses a slight decrease in fluorescence and a slight increase inreflectance. The carious region 58 may be imperceptible or barelyperceptible in either fluorescence image 50 or reflectance image 52,taken individually. Processing apparatus 38 operates upon the image datausing an image processing algorithm as discussed below for both images50 and 52 and provides FIRE image 60 as a result. The contrast betweencarious region 58 and sound tooth structure is heightened, so that acaries condition is made more visible in FIRE image 60.

FIG. 6 shows the contrast improvement of the present invention in aside-by-side comparison with a visual white-light image 54 andconventional fluorescence methods. For caries at a very early stage, thecarious region 58 may look indistinct from the surrounding healthy toothstructure in white-light image 54, either as perceived directly by eyeor as captured by an intraoral camera. In the green fluorescence image52 captured by existing fluorescence method, the carious region 58 mayshow up as a very faint, hardly noticeable shadow. In contrast, in theFIRE image 60 generated by the present invention, the same cariousregion 58 shows up as a darker, more detectable spot. Clearly, the FIREimage 60, with its contrast enhancement, offers greater diagnosticvalue.

Image Processing

As described earlier with reference to FIGS. 5 and 6, processing of theimage data uses both the reflectance and fluorescence image data togenerate a final image that can be used to identify carious areas of thetooth. There are a number of alternative processing methods forcombining the reflectance and fluorescence image data to form FIRE image60 for diagnosis. Copending U.S. patent application Ser. No. 11/262,869,cited earlier, describes a scalar multiplication method for combiningthe fluorescence and reflectance data. In this scalar multipleembodiment, image processing performs the following operation for eachpixel:

(m*F_(value))−(n*R_(value))  (1)

where m and n are suitable multipliers (positive coefficients) andF_(value) and R_(value) are the code values obtained from fluorescenceand reflectance image data, respectively.

Back-scattered reflectance is higher (brighter) for image pixels in thecarious region, yielding a higher reflectance value R_(value) for thesepixels than for surrounding pixels. The fluorescence, meanwhile, islower (darker) for image pixels in the carious region, yielding a lowerfluorescence value F_(value) for these pixels than for surroundingpixels. For a pixel in a carious region, the fluorescence isconsiderably weaker in intensity compared to the reflectance. Aftermultiplying the fluorescence and reflectance by appropriate scalarmultipliers m and n, respectively, where m>n, the scaled fluorescencevalues of all pixels are made to exceed or equal to the correspondingscaled reflectance values:

(m*F _(value))> or =(n*R _(value)).  (2)

Subtraction of the scaled back-scattered reflectance value from thescaled fluorescence value for each pixel then results in a processedimage where the contrast between the intensity values for pixels in thecarious region and pixels in sound region is accentuated, resulting in acontrast enhancement that can be readily displayed and recognized. Inone embodiment, scalar multiplier n for reflectance value R_(value) isone.

Following an initial combination of fluorescence and reflectance valuesas given earlier with reference to the example of expression (1),additional image processing may also be of benefit. A thresholdingoperation, executed using image processing techniques familiar to thoseskilled in the imaging arts, or some other suitable conditioning of thecombined image data used for FIRE image 60, may be used to furtherenhance the contrast between a carious region and sound tooth structure.Referring to FIG. 7, there is shown, in block diagram form, a sequenceof image processing for generating an enhanced threshold FIRE image 64according to one embodiment. Fluorescence image 50 and reflectance image52 are first combined to form FIRE image 60, as described previously. Athresholding operation is next performed, providing threshold image 62that defines more clearly the area of interest, carious region 58. Then,threshold image 62 is combined with original FIRE image 60 to generateenhanced threshold FIRE image 64. Similarly, the results of thresholddetection can also be superimposed onto a white light image 54 (FIG. 6)in order to definitively outline the location of a carious infection.

The choice of appropriate coefficients m and n is dependent on thespectral content of the light source and the spectral response of theimage capture system. There is variability in the center wavelength andspectral bandwidth from one LED to the next, for example. Similarly,variability exits in the spectral responses of the color filters andimage sensors of different image capture systems. Such variations affectthe relative magnitudes of the measured reflectance and fluorescencevalues. Therefore, it may be necessary to determine a different m and nvalue for each imaging apparatus 10 as a part of an initial calibrationprocess. A calibration procedure used during the manufacturing ofimaging apparatus 10 can then optimize the m and n values to provide thebest possible contrast enhancement in the FIRE image that is formed.

In one calibration sequence, a spectral measurement of the light source12 used for reflectance imaging is obtained. Then, spectral measurementis made of the fluorescent emission that is excited from the tooth. Thisdata provides a profile of the relative amount of light energy availableover each wavelength range of interest. Then the spectral response ofcamera 30 (with appropriate filters) or 32 is quantified against a knownreference. These data are then used, for example, to generate a set ofoptimized multiplier m and n values to be used by processing apparatus38 of the particular imaging apparatus 10 for forming FIRE image 60.

While the scalar multiplication method provides improved results overconventional fluorescence imaging, however, there remains some room forimprovement, particularly with respect to edge definition and overallimage quality. One inherent problem with the scalar multiplicationmethod is that multiplication of the weaker fluorescence signal alsoscales up the noise floor. This results in more noise and some loss ofedge definition in the FIRE image.

In an alternative embodiment to the scalar multiplication method, adifferent method, hereafter called the asymmetric illuminance method,can be used. In this method, fluorescence and reflectance are obtainedas separate captures, with more light delivered to the tooth forfluorescence imaging than for reflectance imaging. A significantincrease in excitation light for fluorescence imaging results in ahigher light level in the resulting fluorescence, with a significantlyimproved S/N ratio for the fluorescence image data. By increasing thefluorescence to a high enough level, the fluorescent response can bebrought to a level comparable to or slightly larger than thereflectance, allowing straightforward subtraction to be used forobtaining the difference between the fluorescence and reflectance imagesused for FIRE imaging. It is emphasized that this method does notinvolve up-scaling of the fluorescence signal; thus there is nomagnification of the noise floor.

In practice, there are limitations to the amount of light that can beprovided from a source, particularly one of small size such as would beused for imaging apparatus 10. By also using decreased illuminationduring reflectance capture, comparable fluorescence and reflectancelevels can be achieved not requiring an exceedingly large illuminationincrease for fluorescence capture.

Increased illuminance can be obtained by increasing the drive current tothe LED or other light source that is used for exciting fluorescentemission. In some embodiments (FIGS. 1-4B), the same light source 12 isused for both fluorescent and reflectance imaging. In other embodiments,a separate light source 16 a serves for exciting fluorescence (FIG. 8).Whether the same light source 12 is used for both reflectance andfluorescence imaging or separate light sources 16 a and 16 b are used,each imaging operation may require a separate illuminance level, makingit necessary to capture separate fluorescence and reflectance images atdifferent times. In one embodiment, these images are taken at a fractionof a second apart. Separate filters may be needed, possibly by switchingrapidly into place according to the image that is being captured.

Results from asymmetric illuminance imaging show improvement over thescalar multiplication method of Equation (1). FIG. 9 shows two exampleFIRE images generated from the same tooth. At the left is an image 70obtained using the scalar multiplication method. Image structure isnoticeably darker, especially in the edge features. Also, cariouslesions 86 a and 86 b are overly darkened, failing to show thedistinctive stages of caries development between the two lesions. Image72 on the right, taken using asymmetric illuminance imaging describedwith respect to this second embodiment, shows marked improvement indynamic range and contrast and improved edge definition.

Another alternative embodiment for combining fluorescence andreflectance images takes a different approach from the scalarmultiplication or asymmetric illuminance imaging approaches justdescribed. This “downshifting” or “offset” approach does not riskdistortion of the image data, such as can result from scaling, nor doesit require driving current to high levels. The downshifting imagingmethod can be characterized as keeping image values that are in acertain brightness range and maintaining the input/output ratio of thoseimage values in the processing of the image. In effect, this methodmaintains the input/output relationship and structural integrity of theoriginal data.

The downshifting imaging method operates as follows:

-   1. Obtain the reflectance image and fluorescence image data from the    tooth, each with a suitable illumination level.-   2. In combining the two image data values, subtract an offset    (alternately stated, add a negative offset) from the reflectance    image data, where the offset approximates the difference in    intensity between the image data distributions.

In general, the downshifting imaging method obtains each image valueusing:

(F_(value))−(R_(value)−offset)  (3)

For example:

(F_(value))−(R_(value)−110)  (4)

Implicit in the carrying out of Equation (3) is a clipping operation,where any negative result of the subtraction operation is set to zero.Thus Equation (3) can be more clearly stated as:

Clip{(F_(value))−Clip[(R_(value)−offset)]}  (5)

Here, F_(value) could be obtained from the green color channel, andR_(value) from the blue color channel of the same color capture. Or,F_(value) and R_(value) can be obtained from two separate captures, asin the alternative embodiments previously discussed.

The graphs of FIG. 10 show schematically what the downshifting imagingmethod does to the reflectance value R_(value). The addition of theoffset effectively causes a shift in the effective range of reflectancedata values. The horizontal axis (abscissa) represents the input datacode values. The vertical axis (ordinate) represents output data codevalues. Without any image modification, as shown in the graph at theleft, the input/output mapping 74 has a slope of 1, mapping each inputto an output at the same code value. The graph at the right shows anegative offset 78 applied to input/output mapping 74, resulting in anunused portion 76 of the input data, over the darker region. Outputvalues arc attenuated over the portion of input/output mapping 74 thatis used; however, the same overall relationship (having the same slopeof 1) is maintained; only the overall intensity level is reduced for thereflectance data.

The downshifting imaging method shows pronounced improvement over themultiplicative scaling method for combining fluorescence and reflectanceimage data. FIG. 11 shows two example FIRE images generated from thesame tooth using the same illumination level. At the left is an image 70obtained using the scalar multiplication method. Image 80 on the right,provided using downshifting imaging with an offset, described withrespect to this third embodiment, shows marked improvement in dynamicrange and contrast and improved edge definition. With the downshiftingimaging method, the amount of contrast (i.e., intensity difference)between the carious lesions 86 a and 86 b and the surrounding soundstructures can be adjusted by adjusting the offset value used.

It must be observed that portions of the three different embodimentsdescribed for combining fluorescence and reflectance data can themselvesbe combined to obtain a FIRE image. For example, drive current to lightsource 12 or 16 a/16 b can be adjusted over various settings to obtainfluorescence and reflectance images that have predetermined ranges.Then, some scalar multiplication can be used to adjust these values,combined with some amount of downshifting, using the general adjustmentequation:

(m*F_(value))−(n*R_(value)−offset)  (6)

It is emphasized that the image contrast enhancement achieved in thepresent invention, because it employs both reflectance and fluorescencedata, is advantaged over conventional methods that use fluorescent imagedata only. Conventionally, where only fluorescence data is obtained,image processing has been employed to optimize the data, such as totransform fluorescence data based on spectral response of the camera orof camera filters or other suitable characteristics. For example, themethod of the '2356 Stookey et al. disclosure, cited above, performsthis type of optimization, transforming fluorescence image data based oncamera response. However, these conventional approaches overlook theadded advantage of additional image information that the back-scatteredreflectance data obtains.

It is instructive to observe that spatial correlation of pixels isrequired for combining fluorescence and reflectance values, whetherusing the scalar multiplication method, asymmetric illuminance imagingmethod, or downshifting method just described. That is, relative to thetooth surface, each pixel in the fluorescence image data has acorresponding pixel in the reflectance image data. Thus, it is preferredthat both fluorescence and reflectance images are captured with theimaging probe in the same position and with little or no time betweenimage captures.

Alternate Embodiments

The method of the present invention admits a number of alternateembodiments. For example, the contrast of either or both of thereflectance and fluorescence images may be improved by the use of apolarizing element. It has been observed that enamel, having a highlystructured composition, is sensitive to the polarization of incidentlight. Polarized light has been used to improve the sensitivity ofdental imaging techniques, for example, in “Imaging Caries Lesions andLesion Progression with Polarization Sensitive Optical CoherenceTomography” in J. Biomed Opt., October 2002; 7 (4): pp. 618-27, by Friedet al.

Polarization control can also be advantageously employed as a means tominimize specular reflection. Specular reflection tends to preserve thepolarization state of the incident light. For example, where theincident light is S-polarized, the specular reflected light is alsoS-polarized. Back-scattering, on the other hand, tends to de-polarize orrandomize the polarization of the incident light. Where incident lightis S-polarized, back-scattered light has both S- and P-polarizationcomponents. Using a polarizer and analyzer, this difference inpolarization handling can be employed to help eliminate unwantedspecular reflectance from the reflectance image, so that onlyback-scattered reflectance is obtained.

Referring to FIG. 4A, there is shown an embodiment of imaging apparatus10 that employs a polarizer 42 in the path of illumination light.Polarizer 42 passes linearly polarized incident light. An analyzer 44may be provided in the path of image-bearing light from tooth 20 as ameans to minimize specular reflection component. With this polarizer42/analyzer 44 combination as polarizing elements, reflectance lightsensed by camera 30 or 32 is predominantly back-scattered light, thatportion of the reflectance that is desirable for combination with thefluorescence image data according to the present invention. In the casewhere the illumination light from light source 12 is already linearlypolarized, such as from a laser, polarizer 42 is not needed; analyzer 44would then be oriented with its polarization axis orthogonal to thepolarization direction of the illumination light for rejecting specularreflection.

An alternate embodiment, shown in FIG. 4B, employs a polarizingbeamsplitter 18 (sometimes termed a polarization beamsplitter) as apolarizing element. In this arrangement, polarizing beamsplitter 18advantageously performs the functions of both the polarizer and theanalyzer for image-bearing light, thus offering a more compact solution.Tracing the path of illumination and image-bearing light shows howpolarizing beamsplitter 18 performs this function. Polarizationbeamsplitter 18 transmits P-polarization, as shown by the dotted arrowin FIG. 4B, and reflects S-polarization, directing this light to tooth20. Back-scattering by the tooth 20 structure depolarizes this light.Polarization beamsplitter 18 treats the back-scattered light in the samemanner, transmitting the P-polarization and reflecting theS-polarization. The resulting P-polarized light can then be detected atcamera 30 (with suitable filter as was described with reference toFIG. 1) or color camera 32. Because specularly reflected light isS-polarized, polarization beamsplitter 18 effectively removes thisspecular reflective component from the light that reaches camera 30, 32.

Polarized illumination results in further improvement in image contrast,but at the expense of light level, as can be seen from the descriptionof FIGS. 4A and 4B. Hence, when using polarized light in this way, itmay be necessary to employ a higher intensity light source 12. It isalso of benefit to use polarizing elements having higher transmissionover the wavelength of interest.

One type of polarizer 42 that has particular advantages for use in thepresent application is the wire grid polarizer, such as those availablefrom Moxtek Inc. of Orem, Utah and described in U.S. Pat. No. 6,122,103(Perkins et al.). The wire grid polarizer exhibits good angular andcolor response, with relatively good transmission over the blue spectralrange. Either or both polarizer 42 and analyzer 44 in the configurationof FIG. 4A could be wire grid polarizers. Wire grid polarizingbeamsplitters are also available, and can be used in the configurationof FIG. 4B.

The method of the present invention takes advantage of the way the toothtissue responds to incident light of sufficient intensity, using thecombination of fluorescence and light reflectance to indicate cariousareas of the tooth with improved accuracy and clarity. In this way, thepresent invention offers an improvement upon existing non-invasivefluorescence detection techniques for caries. As was described in thebackground section given above, images that have been obtained usingfluorescence only may not clearly show caries due to low contrast. Themethod of the present invention provides images having improved contrastand is, therefore, of more potential benefit to the diagnostician foridentifying caries.

In addition, unlike earlier approaches using fluorescence alone, themethod of the present invention also provides images that can be used todetect caries in its very early incipient stages. This added capability,made possible because of the perceptible back-scattering effects forvery early carious lesions, extends the usefulness of the fluorescencetechnique and helps in detecting caries during its reversible stages, sothat fillings or other restorative strategies might not be needed.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as described above, and as noted in the appended claims, by aperson of ordinary skill in the art without departing from the scope ofthe invention.

For example, various types of light sources 12 could be used, withvarious different embodiments employing a camera or other type of imagesensor. While a single light source 12 could be used for fluorescenceexcitation, it may be beneficial to apply light from multiple incidentlight sources 12 for obtaining multiple images. Referring to thealternate embodiment of FIG. 8, light source 12 might be a more complexassembly that includes one light source 16 a for providing light ofappropriate energy level and wavelength for exciting fluorescentemission and another light source 16 b for providing illumination atdifferent times. The additional light source 16 b could provide light atwavelength and energy levels best suited for back-scattered reflectanceimaging. Or, it could provide white light illumination, or otherpolychromatic illumination, for capturing a white light image orpolychromatic image which, when displayed side-by-side with a FIREimage, can help to identify features that might otherwise confoundcaries detection, such as stains or hypocalcification.

In one embodiment, a white light image also provides the back-scatteredreflectance data that is used with the fluorescence data for generatingthe FIRE image. To obtain the reflectance image from the white lightimage, a suitable filter is used to transmit a selected portion of thespectrum of reflected light and to block other portions of reflectedlight. Alternately, for a color sensor or camera 32, reflectance data isobtained from one color channel of the white light image, typically notfrom the red channel. While blue portions of the spectrum can be mostfavorably used for reflectance image data, there are advantages to usingthe green spectral range, particularly since the spectral response ofsensors or a color camera is often advantaged for the green portion ofthe spectrum.

In one embodiment, as shown in FIG. 12, FIRE image 64 and white-lightimage 54 display side-by-side on a display monitor 82. FIRE image 64 isgenerally a grayscale image. Alternatively, FIRE image 64 can be tintedwith a greenish coloring. This has been found helpful for the dentist ortechnician operating the imaging apparatus, since it suggestsfluorescence content in FIRE image 64.

Thus, what is provided is an apparatus and method for caries detectionat early and at later stages using combined effects of back-scatteredreflectance and fluorescence.

PARTS LIST

10 imaging apparatus

12 light source

13 diffuser

14 lens

16 a light source

16 b light source

18 polarizing beamsplitter

20 tooth

22 lens

26 filter

28 filter

30 camera

32 camera

34 beamsplitter

38 processing apparatus

40 display

42 polarizer

44 analyzer

50 fluorescence image

52 reflectance image

54 white-light image

58 carious region

60 FIRE image

62 threshold image

64 enhanced threshold FIRE image

70 image

72 image

74 input/output mapping

76 unused portion

78 offset

80 image

82 display monitor

86 a carious lesions

86 b carious lesions

1. A method for forming an enhanced image of a tooth comprising:obtaining fluorescence image data from the tooth by: (i) directing firstincident light toward the tooth, and (ii) sensing fluorescent emissionfrom the tooth; obtaining reflectance image data from the tooth by: (i)directing second incident light toward the tooth, and (ii) sensingback-scattered reflectance light from the tooth; and combining eachpixel in the fluorescence image data with its corresponding pixel in thereflectance image data by: (i) subtracting an offset to the reflectanceimage data value to generate an offset reflectance image data value, and(ii) computing an enhanced image data value according to a differencebetween the fluorescence image data value and the offset reflectanceimage data value, whereby the enhanced image is formed from a resultingpixel array of enhanced image data values.
 2. The method of claim 1further comprising the step of displaying the enhanced image of thetooth.
 3. The method of claim 1 wherein the incident light includeswavelengths between about 300 and 500 nm.
 4. The method of claim 1wherein obtaining the fluorescence image data comprises the step ofusing a green filter.
 5. The method of claim 1 wherein obtaining thereflectance image data comprises the step of using a blue filter.
 6. Themethod of claim 1 wherein the step of obtaining the reflectance imagedata comprises using a camera.
 7. The method of claim 6 wherein thecamera is a color camera.
 8. The method of claim 1 wherein thefluorescence image data and reflectance image data are obtained fromdifferent color planes of a single, full-color image capture.
 9. Themethod of claim 1 wherein the fluorescence image data and reflectanceimage data are obtained from separate image captures.
 10. The method ofclaim 2 wherein the display of the enhanced image has a non-gray colortint.
 11. The method of claim 2 wherein the display of the enhancedimage has a green color tint.
 12. The method of claim 2 wherein the stepof displaying the enhanced image of the tooth tissue comprisessimultaneously displaying an image of the tooth obtained using apolychromatic light source.
 13. The method of claim 1 wherein the stepof directing incident light toward the tooth comprises the step ofenergizing a light source taken from the group consisting of a laser, anLED, and a lamp.
 14. The method of claim 1 wherein obtaining areflectance image comprises directing a polychromatic incident lighttoward the tooth.
 15. The method of claim 1 wherein the reflectanceimage data are obtained from a color plane of a white light image. 16.The method of claim 1 wherein a single light source provides the firstand second incident light.
 17. The method of claim 1 wherein differentlight sources provide the first and second incident light.
 18. A methodfor forming an enhanced image a tooth tissue comprising: obtainingfluorescence image data from a tooth by: (i) directing a first incidentlight toward the tooth, and (ii) sensing fluorescent emission from thetooth; obtaining reflectance image data from the tooth by: (i) directinga second incident light toward the tooth, and (ii) sensing reflectedlight from the tooth, wherein an illuminance of the first incident lightexceeds an illuminance of the second incident light; and combining eachpixel in the fluorescence image data with its corresponding pixel in thereflectance image data to compute an enhanced image data value accordingto a difference between the fluorescence image data value and thereflectance image data value, whereby the enhanced image is formed froma resulting pixel array of enhanced image data values.
 19. The method ofclaim 18 wherein the second incident light is from a polychromatic lightsource.
 20. The method of claim 18 wherein the first incident lightincludes wavelengths between about 300 and 500 nm.